超滤和正渗透过程中有机物对膜污染的研究

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上海交通大学硕士学位论文
ABSTRACT
behavior was also affected by the BSA retention of the membrane. In the FO process, the water and salt flux performance was strongly influenced by the internal concentration polarization (ICP) and also dependent on the membrane orientation. Increasing the foulant concentration, ionic strength and divalent ions concentration in the feed solution as well as salt concentration in the draw solution and lowering the pH promoted the humic acid fouling on the porous support layer of the FO membrane, whereas the cross-flow velocity had little effect on the flux decline with the FO porous support layer towards the feed solution. In addition, humic acid fouling on the FO membrane was less severe with the active layer facing the feed solution than that with the active layer facing the draw solution. UF and FO membrane fouling by the organic macromolecules was affected by the coupled chemical and physical aspects. Despite of different types of organic foulants and membranes, electrostatic repulsion between the foulant-foulant and foulant-membrane was one of the most significant factors affecting fouling. Foulant-membrane interaction dominated the initial flux decline, while the long-term flux behavior was governed by the foulant-foulant interaction. Keywords: ultrafiltration (UF), forward osmosis (FO), bovine serum albumin (BSA), humic acid, limiting flux, concentration polarization
This study investigated the organic macromolecules fouling on the pressure-driven ultrafiltration (UF) membrane and osmotically-driven forward osmosis (FO) membrane. Protein and humic acid, two types of ubiquitous identified membrane organic foulants, were chosen as the model foulants. It was found that the hydrodynamic conditions (initial flux and cross-flow velocity) and feedwater composition (foulant concentration, pH, ionic strength, and divalent ions concentration) played a significant role on the organic fouling of these two types of membranes. During the bovine serum albumin (BSA) ultrafiltration, drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux can not be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point (IEP) of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the IEP. Increasing the ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8, effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient
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上海交通大学硕士学位论文
摘要
响与较长时间内的膜通量的影响刚好相反。由于压缩双电层作用，离子 强度越高时，较长时间的膜通量越低；而短时间的膜通量变化却与超滤 膜对牛血清蛋白分子的截留有关。 在正渗透膜过程中，水通量与盐通量受到内部浓差极化的强烈影响， 并与膜的朝向非常相关，其它条件相同时，当膜的活性层朝向驱动液时 的水通量要比活性层朝向原料液时高。在正渗透膜污染实验中，当活性 层朝向驱动液时，增加原料液中的污染物浓度、溶液离子强度和水质硬 度，降低 pH 值，以及增加驱动液中盐的浓度，都能促进腐殖酸对膜的污 染，而错流速率对膜通量的下降情况几乎没有影响。膜的活性层朝向原 料液时，腐殖酸对膜的污染程度相对于活性层朝向驱动液时减轻。 有机物对超滤膜和正渗透膜的污染是受到化学和物理因素的共同作 用的结果。尽管实验中选用了不同的有机污染物来分别研究两种不同膜 分离过程的膜污染，但是结果发现，污染物与污染物，以及污染物与膜 之间的静电排斥作用是影响膜污染的一个重要因素。污染物与膜之间的 相互作用，支配着起始阶段膜通量的下降，而污染物与膜上已经沉积的 污染物之间的相互作用，则决定着长时间运行过程中膜通量的变化行为。
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关键词：超滤，正渗透，牛血清蛋白，腐殖酸，极限通量，浓差极化
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上海交通大学硕士学位论文
ABSTRACT
The Study of Membrane Fouling during Ultrafiltration and Forward Osmosis by Organic Matters
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